1. Field of the Invention
The present invention relates to a permanent magnet synchronous motor driving system, and a method of testing the permanent magnet synchronous motor driving system.
More specifically, the permanent magnet synchronous motor under the present invention is so devised as to have its leakage current reduced.
The permanent magnet synchronous motor under the present invention is effectively used for an elevator.
2. Description of the Related Art
In general, a permanent magnet synchronous motor (hereinafter referred to as “PM motor” if necessary for convenience sake) has a winding of 2-layer lap type (conventionally used). The PM motor for an elevator is supposed to have an inductive voltage waveform shaped substantially into sinusoid (sine curve) and to reduce torque ripple. Therefore, slot number Q (the number of slots) is, in general, likely to be maximized per pole and per phase.
The PM motor for a gearless hoist has low rotation speed, thus requiring multiple poles. Adopting the 2-layer lap winding into the PM motor for the gearless hoist is likely to further increase the slot number Q. With this, a floating capacitance (electrostatic capacitance) between a winding terminal and ground is likely to increase, thus causing a leakage current with ease. Especially, variably driving the PM motor by means of an inverter is likely to cause the leakage current having high frequency.
Hereinafter described is why variably driving the PM motor by means of the inverter is likely to cause the leakage current.
Recently, high speed power device such as IGBT (=Insulated Gate Bipolar Transistor) has been developed, thus increasing carrier frequency (in other words, switching frequency) of the inverter. Thereby, switching the inverter changes voltage rapidly.
Switching the inverter causes a normal mode voltage (for providing a load current) as well as a common mode voltage. In accordance with the switching state of the inverter, the common mode voltage may change rapidly in such a manner as to form steps. Being totally free from any influence by the current flowing through the load or by an impedance of the load, the common mode voltage can be regarded as a potential of an entire load relative to a standard potential. The common mode voltage has a physical meaning of “zero-phase voltage” which is defined by a coordinate transformation.
FIG. 9 shows a driving system for variably driving a permanent magnet synchronous motor 1 (hereinafter referred to as “PM motor 1”). A rectifier 2 rectifies a three-phase alternating current of a three-phase power source 3 into a direct current. Subsequently, a voltage source PWM inverter 4 supplies the alternating current volts (having variable frequency) to the PM motor 1, where PWM stands for Pulse Width Modulation. With the above operations, the PM motor 1 can be driven variably.
Switching the power device of the voltage source PWM inverter 4 may rapidly change the common mode voltage (which is caused by the voltage source PWM inverter 4) in such a manner as to form steps. Thus, the leakage current is caused to flow from a motor frame to a ground terminal via the floating capacitance of a PM motor winding.
Conventionally, making greater zero-phase impedance, namely, reducing the floating capacitance (electrostatic capacitance) was under consideration in various manners.
It has been proved, however, that taking only the zero-phase impedance into account cannot reduce the leakage currently effectively. More specifically described as follows:
FIG. 1 shows a frequency characteristic of electrostatic capacitance (component of zero-phase) of the PM motor for the gearless hoist, describing measurement of the electrostatic capacitance between three-phase terminals (block) and the ground, according to an earlier technology. An LCR meter is used for measuring the variable frequency, where LCR means Inductance, Capacitance and Resistance. The graph in FIG. 1 has an abscissa depicting the frequency measured in logarithmic scale and an ordinate depicting the electrostatic capacitance measured in logarithmic scale.
Each of a PM motor A and a PM motor B in FIG. 1 shows measured frequency which is stable in a range smaller than or equal to 10.0 kHz. The PM motor A and the PM motor B are, in general, driven by means of the inverter having its carrier frequency smaller than or equal to 10.0 kHz. The thus driven PM motor A and the PM motor B are unlikely to increase the current remarkably, although some leakage current may occur due to ununiformity in the electrostatic capacitance (some are great and others are small).
Notwithstanding the above, the leakage current, as the case may be, occurs in the amount greater than or equal to that which is determined by the electrostatic capacitance measured in FIG. 1.
FIG. 2 shows a frequency characteristic of electrostatic capacitance of the PM motor for the gearless hoist, describing measurement of the electrostatic capacitance between one-phase (U-phase) terminal and the ground. Herein, the PM motor A and the PM motor B in FIG. 2 are those used for the measurement in FIG. 1. The LCR meter is used for measuring the variable frequency. The graph in FIG. 2 has an abscissa depicting the frequency measured in logarithmic scale and an ordinate depicting the electrostatic capacitance measured in logarithmic scale.
In the proximity of the measured frequency of 6.0 kHz in FIG. 2, the PM motor A has its peak electrostatic capacitance attributable to a resonance. On the other hand, in the proximity of the measured frequency of 10.0 kHz in FIG. 2, the PM motor B has its peak electrostatic capacitance attributable to the resonance. The measured frequency corresponding to each of the peak electrostatic capacitances is referred to as “resonant frequency.”
Each of the PM motor A and the PM motor B is driven with the PWM inverter having its main circuit component adopting the IGBT. In general, the PWM inverter has the carrier frequency (in other words, switching frequency) in a range from 5.0 kHz to 15.0 kHz. In sum, the resonant frequency generated between an inductance and a ground electrostatic capacitance of the PM motor A in FIG. 1 is likely to conform with the carrier frequency of the PWM inverter, likewise, the resonant frequency generated between an inductance and a ground electrostatic capacitance of the PM motor B in FIG. 1 is likely to conform with the carrier frequency of the PWM inverter.
The electrostatic capacitance in the low frequency range (smaller than or equal to 1.0 kHz) in FIG. 1 is relatively low. However, the carrier frequency of the PWM inverter conforming with the resonant frequency may greatly increase the electrostatic capacitance, thus increasing the leakage current.
In sum, taking only the zero-phase impedance (namely impedance having component of zero-phase) into account according to the earlier technology is not sufficient for effectively reducing the leakage current. In other words, in addition to the zero-phase impedance, studying a resonant point between the one-phase terminal and the ground is of importance.
Especially, the PM motor for the gearless hoist is, in general, low in speed and low in frequency, thereby becoming great in the number of windings and thereby increasing impedance. With this, the PM motor for the gearless hoist becomes great in ground electrostatic capacitance as well as winding impedance. These features of the PM motor for the gearless hoist are likely to lower the resonant frequency. The thus lowered resonant frequency of the PM motor is likely to conform with the carrier frequency of the inverter.